If you can hide some larger hardware, I'd say go with DMX-512. I wrote an open source library for Linux for speaking DMX-512 over an FTDI USB-to-RS-485 adapter[0], and (shameless plug) have some automation hardware of my own[1] that supports DMX-512 using that library.

It's possible to buy a decent quality 4ch DMX dimmer for ~$30 per channel, and you get instantaneous response rather than the terrible lag of most consumer home automation products.

[0] http://sourceforge.net/projects/dmx485/files/

[1] http://www.nitrogenlogic.com/products/automation_controller....